In the electrophotographic forming of an image, an electrostatic latent image that corresponds to the image is formed on a photoreceptor surface. An electric charging process, in which the photoreceptor surface is uniformly charged, is needed prior to the formation of the electrostatic latent image. The photoreceptor can be charged by one of two schemes: the non-contact charge scheme and the contact charge scheme.
The non-contact charge scheme typically involves the use of a “corotron charger” or a “scorotron charger.” These chargers induce corona discharge, which in turn feeds electric charge to the photoreceptor through the air. Since the charger does not contact the photoreceptor in the non-contact charge scheme, the photoreceptor is less likely to be contaminated or wear out, which are advantages of the scheme. On the other hand, the scheme has a problem that the corona discharge entails ozone and other byproducts.
A recent trend which has emerged due to consideration of the environment is contact chargers which do not involve any corona discharge. Some of them employ a rubber member to contact the photoreceptor. The member is shaped-like a roller, and voltage is applied to it. The roller that includes the rubber member is generally called a charging roller.
The charging roller needs to be set up to exhibit a suitable resistance value so that it can uniformly charge the photoreceptor to a desired electric potential, while preventing current leakage from the charging roller to the photoreceptor. To achieve these effects, for example, documents 1, 2 describe a method for controlling the resistance of the rubber member of the charging roller by adding an electrically conductive agent to the rubber member.
Surface processing technology for the rubber member of the charging roller is described in document 3 to 10. The technology disclosed in the patent documents hardens the surface of a rubber layer made of an epichlorohydrin rubber base material by treating the surface with a solvent containing an isocyanate compound to prevent, for example, ionic conductive agent from leaking through the surface.
The conventional charging roller, however, is likely to cause current leakage from the charging roller to the photoreceptor. Another problem is that the resistance of the charging roller can vary over its life-time.
Document 1: Japanese Unexamined Patent Publication (Tokukai) 2005-43703 (published Feb. 17, 2005)
Document 2: Japanese Unexamined Patent Publication (Tokukai) 2005-121982 (published May 12, 2005)
Document 3: Japanese Unexamined Patent Publication 5-281830/1993 (Tokukaihei 5-281830; published Oct. 29, 1993)
Document 4: Japanese Unexamined Patent Publication (Tokukai) 2000-346051 (published Dec. 12, 2000)
Document 5: Japanese Unexamined Patent Publication (Tokukai) 2001-348443 (published Dec. 18, 2001)
Document 6: Japanese Unexamined Patent Publication (Tokukai) 2002-40760 (published Feb. 6, 2002)
Document 7: Japanese Unexamined Patent Publication (Tokukai) 2002-82514 (published Mar. 22, 2002)
Document 8: Japanese Unexamined Patent Publication (Tokukai) 2004-191960 (published Jul. 8, 2004)
Document 9: Japanese Unexamined Patent Publication (Tokukai) 2004-191961 (published Jul. 8, 2004)
Document 10: Japanese Unexamined Patent Publication (Tokukai) 2006-53544 (published Feb. 23, 2006)